Process for the preparation of glyoxylic acid

ABSTRACT

Glyoxylic acid is prepared by a process facilitating recovery of glyoxylic acid, the process being electrolytic reduction of oxalic acid in an electrolysis cell in which A. THE CATHODE IS SOLID AND METALLIC WITH A HYDROGEN OVERVOLTAGE WHICH IS GREATER THAN THE POTENTIAL FOR THE REDUCTION OF OXALIC ACID TO GLYOXYLIC ACID, B. THE SEPARATING DIAPHRAGM IS A CATION EXCHANGE MEMBRANE, C. THE CATHOLYTE COMPRISES AN AQUEOUS SOLUTION OF OXALIC ACID WHICH IS FREE OF A STRONG INORGANIC ACID, D. THE CATHOLYTE MOVING IN A CLOSED PATH BY BEING PASSED INTO THE CATHODE COMPARTMENT, OVER THE SURFACE OF THE CATHODE, BEING REMOVED FROM THERE AND BEING RETURNED TO THE CATHODE COMPARTMENT, AND E. THE TEMPERATURE OF THE CATHOLYTE IS BETWEEN 0* AND 70*C.

United States Patent [191 Michelet PROCESS FOR THE PREPARATION OFGLYOXYLIC ACID [75] lnventorz Daniel Michelet,

Sain'te-Foy-Les-Lyon, France [73] Assignee: Rhone-Poulenc S.A., Paris,France [22] Filed: Aug. 18, 1972 211 App]. No.: 281,742

[30] Foreign Application Priority Data Aug. 20, 1971 France 7130395 52U.S. Cl. 204/76, 204/77 51 Int. Cl... c070 29/06, 007: 51/40, C07c 53 02[58] Field of Search .1 ..204/75-77 56] 0 References Cited I UNITEDSTATES PATENTS 798,920 9/1905 Von Portheim 204/76 1,013,502

1/191 2 Liebknecht 204776 Primary Examiner-F. C. EdmundsonAtt0rney-.lohn W. Malleyet a1.

[57] ABSTRACT Glyoxylic acid is prepared by a process facilitatingrecovery of glyoxylic acid, the process being electrolytic reduction ofoxalic acid in an electrolysis cell in which a. the cathode is solid andmetallic with a hydrogen overvoltage which is greater than the potentialfor the reduction of oxalic acid to glyoxylic acid,

b. the separating diaphragm is a cation exchange membrane,

c. the catholyte comprises an aqueous solution of oxalic acid which isfree of a strong inorganic acid,

(1. the catholyte moving in a closed path by being passed into thecathode compartmentfover the surface of the cathode, being removed fromthere and being returned to the cathode compartment,'and

e. the temperature of the catholyte is between 0 and 70C.

12 Claims, No Drawings PROCESS FOR THE PREPARATION OF GLYOXYLIC ACID Thepresent invention relates to a process for the preparation of glyoxylicacid by the cathodic reduction of oxalic acid.

The preparation of glyoxylic acid by the cathodic reduction of oxalicacid in sulphuric acid solution has been known since 1903 (German PatentSpecification No. 163,842). This cathodic reduction is carried out at arelatively low temperature (below about 40C), higher temperaturesleading to the formation of glycollic acid [8. Avery et al., Ber. 32,2237 (1899); German Patent Specification Nos. 163,842 and 194,038; H.Nakata, Chem. Abs.,25, 2904].

The presence of a strong inorganic acid (generally sulphuric acid) inthe catholyte is very important; in 1926, Mohrschulz (Zeitsch. FurElektrochem., 32, 449, paragraph 5e) states that the optimumconcentration of sulphuric acid is about 2 to 2.5% and that the use ofconcentrations, which are lower by 0.5%, leads to the formation of onlya minimum amount of glyoxylic acid.

German Pat. Specification No. 204,787, which is a Patent of Addition to194,038 mentioned above describes the use of hydrochloric acid as theelectrolyte but the other characteristics of the process remainessentially the same. I industrially, the isolation of the glyoxylicacid from the reactionmedium is complicated by the presence of thestrong inorganic acid. 1f the reaction mixture consists of an aqueoussolution of oxalic and glyoxylic acids concentrating the solution andcooling it to to 5C precipitates the oxalic acid, leaving a solution ofglyoxylic acid which can be either sold commercially as it is, or can beconcentrated to obtain crystalline glyoxylic acid. 1f the reactionmixture consists of an aqueous solution of oxalic, glyoxylic andsulphuric acids, it is necessary, in addition to the'cooling, to removethe sulphuric acid from the mixture, for example through precipitationwith the aid of alkaline earth hydroxides or salts (German Pat. 163,842,Example 1; Belgian Pat. 757,106, Example 2). German Pat. SpecificationNo. 204,787 describes aiding the removal of this auxiliary electrolyteby replacing thesulphuric acid by hydrochloric acid, which can. beremoved by evaporation. However, hydrochloric acid has otherdisadvantages, principally due to its corrosive character; it is alsodifficult to remove completely by simple evaporation, be cause theevaporation cannot be carried out at a high temperature because of therisks of degrading the glyoxylic acid.

Finally, processes for the preparation of glyoxylic acid by the cathodicreduction of aqueous solution of oxalic acid, which are free fromsulphuric acid, have been described: E. Baur [Zeitsch. fur Elektrochem;

25, 104-5 (1919)] carried out such a process with solid electrodes, butthe chemical yields obtained are not very good, confirming the need forsulphuric acid (see Mohrschulz above). Another process which can useaqueous solutions of oxalic acid which are free from sulphuric acid, isdescribed in Belgian Pat. Specification No. 757,106; this processconsists essentially of carrying out the electrolysis in an electrolyserwhich comprises:

a. a mercury cathode,

b. an anode compartment partially immersed in the cathode mercury,

c. an ion exchange membrane forming one of the walls of the anodecompartment, and A d. a tube immersed in the mercury, enabling catholyteto be supplied in such a way that it forms a thin layer between thecathode mercury and the ion exchange membrane. This process proceedsmore satisfactorily than that of E. Baur, but possesses disadvantageswhich are inherent in the type of apparatus: compact electrolysers .ofthe filter press type cannot be produced, mercury, especially in vapourform presents hazards because of its toxicity; the production ofindustrial electrolysers with horizontal mercury electrodes necessitatesthe use of large amounts of mercury and installations of large surfaceareas, also implying large investments; finally, it is difficult toensure evenness of flow of the thin film of catholyte.

The present invention provides a process for the preparation ofglyoxylic acid by cathodic reduction of oxalic acid, which comprisesperforming the cathodic reduction in an electrolysis cell having acathode compartment containing a catholyte and solid metallic cathodewith a hydrogen overvoltage, which is greater than the potential for thereduction of oxalic acid to glyoxylic acid, and an anode compartmentcontaining an anolyte and an anode, the cathode and anode compartmentsbeing separated by a cation exchange membrane diaphragm, the catholytemoving in a closed path by being passed into the cathode compartment,over the surface of the cathode, being removed from there and returnedto the cathode compartment, the catholyte being at a temperature of 0 to70C, preferably 5 to 35C and comprising an aqueous solution of oxalicacid, which is free of strong inorganic acid, but can containglyoxylic-acid.

The cathode, and anode and the separating diagraphm are preferablycoated in parallel planes; advantageously, several of the elementaryelectrolysis cells can be combined in the manner of a filter press.

Examples of material forming the cathode surface are lead; solidamalgams of lead; alloys of lead with silver, antimony, tin or bismuth;and cadmium. The cathode interior and surface are usually of the samematerial, e.g., a lead plate, but can be different, e.g., a leadplate'with a load amalgam surface.

Anode of the electrolysis cells usually consists of a' solidelectrically conducting material which is electrochemically stable inthe anolyte and under the operating conditions considered. Examples ofsuch materials are metals and metalloids such as platinum, platinisedtitanium, graphite, lead and its alloys, particularly with silver,antimony or tin.

Any known cation exchange membrane can be used to separate the catholytefrom the anolyte, but membranes of the homogeneous type and membranes ofthe heterogeneous type are preferred. Thes membranes can optionally bereinforced with a screen. For carrying out electrolysis operations overa long period, it is naturally preferred to use membranes, which do notswell and which are stable to the action of the various constituents ofthe catholyte and the anolyte. Examples of such membranes are thosedescribed in the specifications of US. Pat. No. 2,681,320 and FrenchPat. Nos. 1,568,994, 1,575,782, 1,578,019, 1,583,089 and 1,584,187. Thepermeation selectivity of the membranes used (defined in and measured asin French Pat.

Specification No. l,584,l87) is preferably greater than The catholytecan consist essentially of water and oxalic acid with, optionally,glyoxylic acid; the catholyte can contain oxalic acid without glyoxylicacid only at the start of electrolysis; in the same way, the catholytecan contain glyoxylic acid without oxalic acid only at the end ofelectrolysis. The concentrations of oxalic and glyoxylic acids can beeither constant when the reaction is carried out continuously, orvariable when the reaction is carried out discontinuously or at thestart of a continuous operation. In all cases, the concentration ofoxalic acid is less than the saturation concentration at the temperatureof electrolysis; generally, this concentration is greater than 2% byweight, and preferably greater than 3% when the current density is high,these values relating particularly to the constant concentration whenthe reaction is carried out continuously and to the final concentrationwhen the reaction is carried out discontinuously. The concentration ofglyoxylic acid is usually between 3 and 25% by weight, and preferablybetween and 15% by weight, these values relating particularly to theconstant concentration of glyoxylic acid when the reaction is carriedout continuously and to the final concentration of this acid when e i isri d Q Qi QP "E9". lL-

The catholyte can also contain reaction by-products in small amounts,e.g., generally less than 1% by Wei An aqueous acid solution ispreferably used as the anolyte, though any other anolyte capable ofproviding electrical conductivity between the two electrodes can beused. Aqueous solutions of sulphuric or phosphoric acids are usuallyemployed in a concentration generally of 0.l to 5 m ols/litre, and preferably 0 5 to 2 mols/litre.

The current density at the cathode is preferably 3 to 50 A/dm especiallylO to 3 5 A/dn 1 g g The flow of the catholytein a closed circuit isusually achieved by means of a pump; the circuit can in addition containattached devices such as a heat exchanger or an expansion vessel. Theexpansion vessel enables oxalic acid to be added to the catholyte andalso some catholyte to be withdrawn in order to extract the glyoxylicacid.

At least one spacer is preferablypresent iii'i'fi'iiitfie and/or cathodecompartments; these spacers serve to prevent deformations of the cationexchange membrane, and contact between this membrane and the electrodesand also help to render the catholyte of uniform concentration. Thesespacers are generally manufactured from synthetic polymers which arechemically inert and which do not conduct electricity; they can be madein the form of interlaced, interwined, knotted or welded yarns (e.g.,woven fabrics, grids or nets) or they can be in the form of platespossessing holes or grooves. In practice, these spacers are orientedalong planes which are parallel to those of the electrodes and theseparating diaphragm.

In the absence of a spacenThe s'b'tVifififi' speed of the catholyte(i.e. the speed in the cathode compartment, assumed to contain nospacer)is usually greater than 1 cm/second, and preferably greater thancm/second.

The catholyte is preferably degassed in the expansion 5 vessel with theaid of a stream of inert gas, e.g., nitrogen.

At the end of electrolysis, the glyoxylic acid is isolated by the knownmeans, especially in the manner described above by concentrating andcooling the catho- 10 lyte, optionally under reduced pressure. Thecooling,

which results in precipitation of the oxalic acid, is carried out attemperatures which are generally below 8, and preferably not more than5C; the degree of concentration and the cooling temperature naturallyvary according to the degree of purity desired for the glyoxwhich do notcontain strong inorganic acid, e.g., sulphuric or hydrochloric acid andhence facilitate work up and recovery of the glyoxylic acid: it allowselectrolysis cells to be produced which are compact and easy todismantle; it allows the gases which are produced at the anode,especially oxygen, which are capable of creating regions of lower oreven zero current density, to be removed easily. It makes it possible touse high current densities and to achieve easily the supply ofelectricity in series between the various elementary electrolysis cellsin an assembly of several cells; it makes it possible to use cells withvertical electrodes; finally, due to the constant geometrical shape ofthe preferred electrolysis cells, the anolyte and the catholyte can becirculated very rapidly, enabling lower concentrations of oxalic acid tobe employed and, as a result, better degrees of conversion in continuousoperation to be obtained.

The following Examples illustrate the invention. The chemical yieldsindicated are yields of glyoxylic acid relative to the oxalic acidconverted. Concentrations of solutions expressed as a percentage denote,unless otherwise stated, the number of grams of solute per 100 cm ofsolution; however, these concentrations in g/ 100 cm differ onlyslightly from concentrations in (weight/weight) because the solutionsemployed in the Examples generally have a density of about 1.

EXAMPLE l The reduction of oxalic acid to glyoxylic acid is carried outin an electrolysis cell possessing the following characteristics: bothelectrodes are rectangular plates of lead, the usable surface area ofeach of which is 0.8 dm the the cation exchange membrane separatinganode and cathode compartments is of the heterogeneous type consistingof a cross-linked, sulphonated styrene/divinylbenzene copolymer,dispersed in a matrix of vinyl chloride polymer, and it is reinforcedwith a woven fabric of polyethylene glycol terephthalate. Thesubstitution resistance of the membrane is 7 9 cm (measurement made in0.6 M KCl solution) and its permeation selectivity is 77.5%; thedistance from the electrode to the membrane is 3 mm; two pumps cause thecatholyte and the anolyte to circulate in the electrolysis cell; and theanolyte and the catholyte circulating in external circuits each containan expansion vessel equipped with supply and removal pipe- ,lines; thecatholyte circuits also contain a cooling The electrolysis conditionsare as follows:

current density: 17.5 A/dm voltage: 5.2 V

temperature: betweeen 25 and 28C linear speeds of the anolyte and thecatholyte over their respective electrodes: about 1.5 m/second duration:6 hours anolyte: sulphuric acid in a 10% aqueous solution initialconcentration of oxalic acid in the catholyte:

initial concentration of glyoxylic acid in the catholyte: 3,48%

volume of the catholyte: 1.3 l

oxalic acid added to the catholyte: 0.4 I/hour of a 9.65% aqueoussolution of oxalic acid.

Sufficient catholyte is removed to keep constant the volume of thecatholyte which is circulating.

At the end of the electrolysis, the catholyte which is still circulatingand the catholyte, which was removed during the electrolysis arecombined, and a solution is obtained in which the concentration ofoxalic acid is 4.5% and that of glyoxylic acid is 3.8%.

Current yield: 83%

Chemical yield: 93.6%

EXAMPLE 2 An electrolysis is carried out in the same apparatus as inExample 1, under the following conditions:

current density: 17.5 A/dm voltage: 5.2 V temperature: between 25 and28C linear speeds of the anolyte and the catholyte over their respectiveelectrodes: 1.5 m/second duration: 9 hours 30 minutes initial volume ofthe catholyte: 1.3 l initial concentrations in the catholyte:

of glyoxylic acid: 8.9% of oxalic acid: 5.13% oxalic acid added to thecatholyte: 0.187 l/hour of a 15.7% solution of oxalic acid. Sufficientcatholyte is removed to keep its volume constant. I

At the end of the experiment,-the catholyte, which is still circulating,and the catholyte, which was removed during the electrolysis, arecombined; 3.2 l of a solution are thus obtained,.which contains 8.6% ofglyoxylic acid and 4.3% of oxalic acid.

Electrical yield: 86% Chemical yield: 92.7% Degree of conversion: 69.5%Crystalline glyoxylic acid is prepared from the above solution by thefollowing method. This solution is concentrated at 30C in vacuo, cooledto 0C and filtered;

the filtrate has a glyoxylic acid content of 45% (weight/weight) whilstthe precipitate has an oxalic acid content of 99.5% (weight/weight).These operations are repeated until a solution containing 60%(weight/weight) of glyoxylic acid is obtained. This solution is storedfor 24 hours at 5C; a precipitate forms which is filtered off: whitecrystals of 95.7% pure glyoxylic acid monohydrate are thus obtained.

Example 3 The reduction of oxalic acid to glyoxylic acid is carried outin an electrolysis cell similar to that of Example 1, but the usablesurface area of each electrode of which is 2.5 dm

The electrolysis conditions are as follows:

current density: 14 A/dm voltage: 4.55 V

temperature: 2122C linear speed of the electrolytes over the electrodes:

about 1 m/second catholyte introduced initially: 7 l of a 3.64% strengthsolution of oxalic acid.

This solution is electrolysed for 7 hours 15 minutes, adding freshcatholyte at a rate of 0.542 l/hour with a 14.08% solution of oxalicacid and simultaneous removal of catholyte sufficient to keep the volumeof the catholyte constant. The following catholyte then contained 3.07%oxalic acid and 3% glyoxylic acid.

Electrolysis is then carried out continuously for 24 hours, supplyingfresh catholyte at a rate of 1.14 l/hour with a 8.5% solution of oxalicacid and simultaneous removal in corresponding amounts as before. At thestart of this second stage of electrolysis, the volume of the catholytewas reduced to 7 1; this second stage is a stage of continuous operationin that the concentration of glyoxylic acid remains substantiallyconstant (about 3%).

The catholyte at the end of the experiment contains 4.28% oxalic acidand 3.03% glyoxylic acid.

At the end of the experiment, the catholyte which is still circulatingand the catholyte which has been removed are combined to give a totalvolume of 37.85 1, containing 1,200 g glyoxylic acid and 1,422 g oxalicacid. Electrical yield: 79.6% Chemical yield: 85.5%.

Example 4 In this experiment, the apparatus is the same as in Ex ample3.

Electrolysis conditions:

current density: 14 A/dm voltage: 4.5 V

temperature: 20C

linear speed of the electrolytes over the electrodes: 1 m/second. Thecatholyte is continuously freed from gas in the expansion vessel by astream of nitrogen of about l/hour.

Catholyte originally introduced: 6.3 l of a 3.82% strength solution ofoxalicacid.

This'solution is electrolysed for 7 hours, supplying it at a rate of0.495 l/hour with a 15.85% solution of oxalic' acid with removal of thecatholyte sufficient to keep its volume constant.

Electrolysis is then carried out continuously for 34 hours 30 minutes(the concentrations remaining substantially constant), supplying thecatholyte at the rate of 0.810 l/hour with 10.58% oxalic acid. Duringthis second period, the volume of the catholyte is kept constant at 9 l.The catholyte at the end of the experiment contains 4.8% glyoxylic acidand 3.79% oxalic acid.

At the end of the experiment, the catholyte, which is still circulating,and the catholyte, which was removed during the electrolysis, arecombined to give 38.72 1 of solution containing 1,727 g glyoxylic acidand 1,400 g oxalic acid. Electrical yield: 86.4% Chemical yield: 89.3%

EXAMPLE 5 In this experiment, the assembly is the same as in Example 3,but the cathode is a lead alloy containing 5% of silver.

Electrolysis conditions:

current density: 14 A/dm voltage: 4.6 V

temperature: 20C

linear speed ofthe electrolytes over the electrodes:

1 m/second degassing of the catholyte by means of nitrogen: 200

l/hour catholyte introduced initially: 6 l of a 3.64% solution of oxalicacid.

This solution is electrolysed for 7 hours 40 minutes, supplying freshcatholyte at a rate of 0.495 l/hour with a 16% solution of oxalic acidand simultaneous removal as in previous Examples. Electrolysis is thencarried out under conditions of continuous operation for 10 hours,supplying the catholyte at a rate of 0.790 l/hr. with a 10.7% solution.During this period the volume of the catholyte is kept constant andequal to 9 1.

At the end of the experiment, the catholyte contains 4.22% glyoxylicacid and 4.4% oxalic acid.

After having combined the catholyte,which is still circulating and theliquid, which was removed during the experiment, a total volume of 18.651 is obtained oontaining 734 g glyoxylic acid and 801 g oxalic acid.Electrical yield: 86.5% Chemical yield: 97.8%

Example 6 The apparatus described in Example 3 is used with thefollowing electrolysis conditions current density: 25 A/dm voltage: 5.45V

temperature: 20",C

speed of the electrolytes over the electrodes: 1 m/second degassing ofthe catholyte with nitrogen at a rate of 200 to 300 l/hour catholyteintroduced initially: 6.6 l ofa 3.7% solution of oxalic acid.

This solution is electrolysed for 5 hours minutes, supplying thecatholyte with 0.8 l/hour of a solution containing 16.7% by weight ofoxalic acid and removal to constant volume as in previous Examples.Electrolysis is then contained for 14 hours 45 minutes, supplying thecatholyte with 1.460 l/hour ofa 10.65% solution of oxalic acid. Duringthis last period, the volume of the catholyte is kept at 9 1. At the endof the experiment, the catholyte contains 4.64% glyoxylic acid and 4.18%oxalic acid.

The catholyte, which is still circulating, and the catholyte, which wasremoved are combined and 34.4 1 of solution are obtained containing1,524 g glyoxylic acid and 1,382 g oxalic acid. Electrical yield: 88.3%Chemical yield: 96.3%.

EXAMPLE 7 The assembly described in Example 1 is used.

The cathode is a plate of pure lead which has been amalgamated at thesurface with mercury (0.5 cm Hg for a total surface area of 2 dm). Theusable cathode surface area is 0.8 dm

Electrolysis conditions:

current density: 12.5 A/dm voltage: 4.3 V

temperature: 25C

speed of the electrolytes over the electrodes: 1.5

m/second catholyte introduced initially: 1.5 l of a 4.65% solu j tion ofoxalic'acid.

This solution is electrolysed for 8 hours suppying the catholyte with17.85 g/l of oxalic acid and removal to constant volume as in previousExamples. At the end of the experiment, the volume of the catholyte is1.7 1 containing 5.8% glyoxylic acid and 3.93% oxalic acid. Electricalyield: 89% Chemical yield: 82.4%.

EXAMPLE 8 in this embodiment, an assembly of the filter press type isproduced, with 4 cells of 2.5 dm each one being similar to thatdescribed in Example 3.

These 4 cells are supplied with electrolyte in parallel, and current inseries, two by two.

current density: 15 A/dm voltage: 9.24 V (for 2 cells supplied inseries) temperature: 2830C circulation speed of the electrolytes overthe electrodes: 1 m/second catholyte introduced initially: 8 l of a3.46% solution of oxalic acid.

Electrolysis is carried out for 7 hours 35 minutes, supplying 0.660l/hour of a 36.7% solution of oxalic acid, and removal to'constantvolume as in previous Examples.

23 mols of oxalic acid are used as starting material and 7.5 mols ofoxalic acid are recovered unused; hence 15.5 moles oxalic acid reacted.15.2 mols glyoxylic acid are produced. Electrical yield: 75% ChemicalYield: 98.5%.

1 claim:

1. Process for the preparation of glyoxylic acid by cathodic reductionof oxalic acid, which comprises performing the cathodic reduction in anelectrolysis cell having a cathode compartment containing a catholyteand a solid metallic cathode which a hydrogen overvoltage, which isgreater than the potential for the reduction of oxalic acid to glyoxylicacid, and an anode compartment containing an anolyte and an anode, thecathode and anode compartments being separated by a cation exchangemembrane diaphragm, the catholyte moving in a closed path by beingpassed into the cathode compartment, over the surface of the cathode,being removed from there and returned to the cathode compartment, thecatholyte being at a temperature of 0 to C and comprising an aqueoussolution of oxalic acid, which is free of strong inorganic acid.

2. A process according to claim 1, wherein in the cathode at least thesurface thereof consists of acomposition selected from the groupconsisting of cadmium, lead, a solid amalgam of lead, and an alloy oflead with a metal selected from the group consisting of silver,antimony, tin and bismuth.

3. A process according to claim 1 wherein the concentration of oxalicacid in the catholyte is greater than 2% by weight.

4. A process according to claim 3, wherein catholyte contains between 3and 25% by weight of glyoxylic acid.

5. A process according to claim 3, wherein the cathode current densityis 3 to 50 Aldm 6. A process according to claim 1, wherein oxalic acidis added continuously to the catholyte and catholyte is withdrawncontinuously in order to extract the glyoxylic acid from it.

7. A process according to claim 1, wherein the catholyte having at leasta liquid phase is withdrawn, the liquid phase is cooled to a temperatureof at most 5" C to deposit a precipitate and the precipitate is filteredoff.

8. A process according to claim 1, wherein the anolyte circulates in amanner similar to that of the catholyte, so that the pressure on eitherside of the cation exchange membrane is substantially the same.

9. A process according to claim 1, wherein at least one of the anode andcathode compartments contains a spacer.

10. A process according to claim 9, wherein the cathode compartmentcontains a spacer and the apparent speed at which the catholytecirculates in the cathode compartment is greater than 1 cm/sec.

11. A process according to claim 1 wherein the cathode compartment isfree from any spacer and the speed at which the catholyte circulates inthe cathode compartment is greater than 10 cm/sec.

12. A process according to claim 1, wherein the cathode is selected fromthe group consisting of lead, an alloy of lead with 5% silver, and leadwith an amalgamated lead surface, the cathode current density is 12.5 25Aldm the cation exchange diaphagm is of the heterogeneous typeconsisting of a cross-linked sulphonated styrene divinylbenzenecopolymer dispersed in a matrix of vinyl chloride polymer, the catholyteand anolyte are both circulated outside the cell, the speed at which thecatholyte and anolyte pass over the cathode and anode respectively iscm/sec, the catholyte temperature is 20 30C, the catholyte is degassedwith nitrogen, and oxalic acid is added continuously to the catholyteand catholyte is withdrawn continuously in order to extract theglyoxylic acid from it.

2. A process according to claim 1, wherein in the cathode at least thesurface thereof consists of a composition selected from the groupconsisting of cadmium, lead, a solid amalgam of lead, and an alloy oflead with a metal selected from the group consisting of silver,antimony, tin and bismuth.
 3. A process according to claim 1 wherein theconcentration of oxalic acid in the catholyte is greater than 2% byweight.
 4. A process according to claim 3, wherein catholyte containsbetween 3 and 25% by weight of glyoxylic acid.
 5. A process according toclaim 3, wherein the cathode current density is 3 to 50 A/dm2.
 6. Aprocess according to claim 1, wherein oxalic acid is added continuouslyto the catholyte and catholyte is withdrawn continuously in order toextract the glyoxylic acid from it.
 7. A process accordiNg to claim 1,wherein the catholyte having at least a liquid phase is withdrawn, theliquid phase is cooled to a temperature of at most 5*C to deposit aprecipitate and the precipitate is filtered off.
 8. A process accordingto claim 1, wherein the anolyte circulates in a manner similar to thatof the catholyte, so that the pressure on either side of the cationexchange membrane is substantially the same.
 9. A process according toclaim 1, wherein at least one of the anode and cathode compartmentscontains a spacer.
 10. A process according to claim 9, wherein thecathode compartment contains a spacer and the apparent speed at whichthe catholyte circulates in the cathode compartment is greater than 1cm/sec.
 11. A process according to claim 1 wherein the cathodecompartment is free from any spacer and the speed at which the catholytecirculates in the cathode compartment is greater than 10 cm/sec.
 12. Aprocess according to claim 1, wherein the cathode is selected from thegroup consisting of lead, an alloy of lead with 5% silver, and lead withan amalgamated lead surface, the cathode current density is 12.5 - 25A/dm2, the cation exchange diaphagm is of the heterogeneous typeconsisting of a cross-linked sulphonated styrene divinylbenzenecopolymer dispersed in a matrix of vinyl chloride polymer, the catholyteand anolyte are both circulated outside the cell, the speed at which thecatholyte and anolyte pass over the cathode and anode respectively is100 - 150 cm/sec, the catholyte temperature is 20 - 30*C, the catholyteis degassed with nitrogen, and oxalic acid is added continuously to thecatholyte and catholyte is withdrawn continuously in order to extractthe glyoxylic acid from it.